1. Field of the Invention
The present invention relates to digital printing devices, and, in particular, to methods and apparatus for enhancing color saturation for use in halftoning operations that convert a color input to a binary output suitable for printing.
2. Description of the Related Art
Most computer-driven printing devices which generate hard copy, such as laser, dot-matrix and ink-jet printers, print in a binary fashion--the output medium is divided into an array of picture elements or "pixels" and the devices can either print a small colored dot at each pixel location or leave the pixel location blank. In the case of monochrome printers, all of the dots are printed with a single color whereas with color printers a dot color is chosen from a small set of colors. In any case, the dot itself has a uniform color so that the resulting output consists of an array of colored and blank pixels.
Pictorial images such as those produced by photographic techniques or by computerized imaging systems, by contrast, are continuous in tonality. If a monochrome image is divided into pixels, each pixel exhibits a "grayscale" color whose tonal value falls within a range of tonal values. Similarly, if a color image is divided into pixels, each pixel exhibits a hue and an intensity both of which fall in ranges. In order to reproduce such "continuous-tone" images by means of electronic printing, the images must therefore be converted into a form which is suited to the characteristics of the printing device, generally a binary format. This conversion process, which may take many forms, is generically referred to as "halftoning." Although a halftone image actually consists solely of a spatial pattern of binary pixels (colored or blank dots), the human visual system integrates this pattern to create an illusion of a continuous-tone image.
During the printing process, the image to be printed is divided into a series of pixels and the value of the image in each pixel is quantized to produce a multi-bit digital word which represents the tonal value of the pixel. The image is thus converted to a stream of digital words which are provided to the printing device. In order to convert the format of each word into a format suitable for reproduction on the digital device, halftoning is performed on the digital word stream during a process called preprocessing. Numerous halftoning techniques have been developed and refined over the years. In their simplest form, such techniques compare the value of each digital word with a threshold level, and generate a binary output pixel value depending on the relative values.
For example, a digital scanner processing a continuous-tone monochrome image might generate a stream of multi-bit words representing the detected light intensities. Commonly, the numerical value of these words ranges from 0 to 255, corresponding to a 256-level gray scale or an eight-bit word. If such a digital word stream is to be reproduced on a binary printing device, the halftoning process compares the scanner output words with a either a single threshold value or an array of threshold values to produce the required binary output pixel stream. In such a system, each 8-bit scanner word has effectively been compressed into a single-bit output word.
Color images are typically processed by separating each color into one or more color components or "primaries" whose superposition generates the desired color. Generally, three primary colors (either the conventional "additive" primary colors--red, green and blue or the conventional "subtractive" primary colors--cyan, magenta and yellow) are used. A digital scanner processing a continuous-tone color image might generate a stream of multi-bit words for each of the three color components (usually the additive primary colors). Commonly, the numerical value of these words also ranges from 0 to 255, corresponding to 256 intensity levels or an eight-bit word. Thus, each colored pixel is represented by 3 eight-bit words or 24 bits total.
The digital word stream corresponding to a colored image is halftoned by comparing the eight-bit word for each color component with a threshold value in the same manner as monochrome processing. The color components are processed separately so that the three 8-bit scanner words are compressed into a three-bit output word which is eventually printed as three dots--each dot being printed in one of the primary colors.
Theoretically, it is possible to reproduce the full range of printable colors using varying intensities of the three primary colors and superimposing the three dots. For example, if the maximum intensity of each color is printed and the three dots are superimposed, the resulting dot should appear as black. In practice, however, superimposing or overprinting 100% of each primary color produces a gray-brown colored dot and completely saturates the paper with ink so generally black is added as a fourth color. Thus, the colors used are red, green, blue and black (RGBK) or cyan, magenta, yellow and black (CMYK). The black value is also subject to thresholding in order to generate a halftoned output so that the final output to the printer comprises four bits--one bit for each of the three primary color values and black (R, G, B and K).
The effect of the halftoning operation is to convert a solid color or gray area into a pattern of colored or black dots interspersed with white spaces or "dots" (where no dots have been printed). The viewer's eye integrates the pattern of dots and produces the perception of a desired color or gray shade. However, the presence of the white dots in areas which should consist entirely of a color reduces the perceived intensity, or saturation, of the color so that the color appears somewhat "washed out" to an objective observer as compared to the original image.
Accordingly, it is an object of the present invention to provide an apparatus and a method for improving the perceived color saturation of halftoned color images.
It is another object of the present invention to provide an apparatus and a method for improving the perceived color saturation in halftoned color images without introducing visual artifacts into the halftoned image.
It is yet a further object of the present invention to provide an apparatus and a method for improving the color saturation of halftoned color images which apparatus and method can be easily and inexpensively implemented.